The long-range goal of this project is to determine the effect of various repair-deficient strains on the process of mutation-induction at specific loci in Neurospora crassa. Using the ad-3 test system in a two-component heterokaryon, specific locus mutations can be recovered which result from both point mutations and chromosome deletions; this latter class is haplo-lethal and cannot be recovered in experiments on haploid strains. Various studies have shown that with variation in dose-rate with x-rays, photo-reactivation after UV, etc., the lesions giving rise to these two classes of mutations are qualitatively different. The objective of this work is to determine whether upr-1, uvs-2, uvs-3, uvs-5, uvs-6 show a differential effect on the two classes of ad-3 mutations.